The present invention relates generally to external-cavity, surface-emitting semiconductor lasers. It relates in particular to an external-cavity, surface-emitting semiconductor laser having a resonator formed between a movable polarization selective cavity-mirror and a polarization selective mirror.
In optical communications systems, information is transmitted along an optical fiber as a modulated beam of light. In one preferred optical communication arrangement, wavelengths for the light beam are in a range between about 1.513 micrometers (xcexcm) to about 1.565 xcexcm, corresponding to a frequency range from about 196,000 gigahertz (GHz) to about 192,000 GHz. In a scheme referred to as dense wavelength-division-multiplexing (DWDM) the frequency range is partitioned into 40 channels at 100 GHz intervals. A trunk optical fiber may carry up to 40 different beams at 40 different wavelengths, one corresponding to each channel. The different-wavelengths (optical-carrier) beams are generated by InGaAsP edge-emitting diode-lasers, one for each channel. The output of each laser is modulated to encode the information to be transmitted onto the laser-beam provided by the laser. Communications channels are separated from or added to the trunk optical fiber by wavelength-selective couplers.
An optical communications system having such close channel spacing would benefit from a laser that could be rapidly and accurately tuned from the wavelength of one channel to the wavelength of another. One suitable laser type for this purpose is an external-cavity, optically-pumped, surface-emitting semiconductor laser (OPS-laser). Such a laser has a resonator (resonant cavity) formed between two mirrors. One of the resonant cavity mirrors is an integral part of a multilayer structure including a semiconductor multilayer, surface-emitting gain-structure. The mirror can be formed from metal, dielectric, or semiconductor layers or combinations thereof. The other resonant cavity mirror is external to and spaced apart from the gain-structure. This mirror is partially transmissive and is used as an output-coupling mirror. The mirror is usually formed from dielectric layers, semiconductor layers or combinations thereof.
The emitting wavelength of the laser depends on the materials of the gain-structure and the optical spacing between the first and second mirrors. The gain-structure provides gain only in a limited range of wavelengths. This range is generally referred to as the gain bandwidth. By way of example, an InGaAsP OPS-laser having a nominal emitting wavelength of about 1.550 xcexcm has a gain bandwidth of about 0.035 xcexcm. The second mirror can be made movable for varying the spacing, thereby tuning the laser to vary the emitting wavelength within the gain bandwidth. (See, for example, U.S. Pat. Nos. 5,291,501; 5,572,543 and 6,154,471, incorporated herein by reference.)
A preferred arrangement of such a tunable external-cavity semiconductor laser is one in which the nominal spacing between the mirrors is sufficiently small that the separation between possible resonant wavelengths of the cavity is greater than the gain bandwidth. This is often referred to by practitioners of the art as a short-cavity OPS-laser. The cavity length may be about 30 xcexcm for a OPS-laser having a nominal emitting wavelength of about 1.550 xcexcm. The short cavity provides that the laser can emit at only one wavelength within the gain bandwidth for any variation of laser spacing within one half-wavelength of the nominal spacing. Accordingly, no other mode selection device is necessary. Such a laser is also very compact, enabling a number of such lasers to be assembled in a compact array.
In longer cavity lasers, a tilt-tunable etalon, a birefringent filter, or a diffraction grating filter is typically used to limit the number of possible oscillating modes. Such devices also cause the laser to oscillate in a plane-polarized mode. An edge-emitting laser inherently has polarized sensitive gain. Plane-polarized operation is advantageous in optical communications systems that include polarization sensitive devices such as Faraday rotators, and multilayer dielectric mirrors used at non-normal incidence. In a short cavity OPS laser there is insufficient space between the external mirror and the gain-structure to include a tilted etalon or a birefringent filter and the laser oscillates in a minimally defined polarization mode, often with only minimal power difference between two eigen polarizations. A laser without a clearly defined polarization is more susceptible to feedback in the orthogonal polarization. This shortcoming needs to be overcome to improve the potential of the short-cavity, external resonator OPS-laser.
In one aspect, the present invention is directed to a mirror for reflecting light at a lasing wavelength and transmitting light at an optical pump light wavelength. The optical pump light wavelength is the shorter of the wavelengths. The mirror includes a peripherally supported membrane. The membrane has a grating on an outer surface thereof. The grating includes a regular array of spaced-apart parallel strips of a first material having a first refractive index. The grating surmounts at least one optical interference layer of uniform thickness. The optical interference layer can be a layer of the first material, or a layer of a second material having a second refractive index different from the first refractive index. The grating strips are characterized as having a width and a height, and the grating has a grating period defined as the distance between adjacent ones of the parallel strips. The grating period is less than the pump light wavelength. The first material, any second material, the grating width and height, the ratio between the grating width and period, and the thickness of the at least one and any other uniform-thickness layers are selected such that the second mirror has a different specular reflectivity for light having the lasing wavelength, normally incident in first and second polarization planes oriented respectively parallel and perpendicular to the grating strips, and such that the mirror has a transmissivity greater than 50% for pump-wavelength light polarized in any one of the polarization planes.
The term specular reflectivity (or simply reflectivity) as used herein can be alternatively defined as the zero-order diffraction efficiency, where this term refers to the zero order diffraction efficiency of the grating and any other optical interference layers as a whole. The term xe2x80x9ctransmissivityxe2x80x9d refers to the zero-order or specular transmissivity. Reflectivity, transmissivity and diffraction efficiency are specified herein alternatively as a percentage or a decimal ratio where 1.0 is 100%.
Preferably the specular reflectivity for the lasing wavelength is between about 90% and 99% in one of the polarization planes, and is at least about 1% less in the other polarization plane, and the transmissivity of the mirror for pump-wavelength light is greater than 50% (0.50 in decimal notation) in some polarization orientation.
The at least one optical interference layer may be a layer of the first material, i.e., the material of the grating strips. Alternatively, the at least one optical interference layer is a layer of a material having a refractive index different from that of the grating strips.
In one preferred embodiment of the inventive grating-membrane mirror there is only one interference layer having the same refractive index as that of the grating strips The grating strips and mirror in this embodiment may be considered as a single grating comprising the spaced apart strips and a uniform thickness portion. The specular reflectivity for laser light in either of the polarization orientations is greater than would be provided by the optical interference layer (uniform thickness portion of the grating) in the absence of the grating strips. In another preferred embodiment of the inventive grating membrane mirror a grating including a uniform thickness portion surmounts four other optical interference layers. The first-wavelength specular reflectivity in either of the polarization orientations is less than would be provided by the uniform thickness portion of the grating and the four optical interference layers in the absence of the grating strips. In calculated examples of each embodiment, a lasing wavelength reflectivity of about 98% for wavelengths between about 1.530 and 1.565 xcexcm is achieved together with greater than 70% transmissivity for 0.980 xcexcm plane-polarized (TM) pump light. Reflectivity for TE polarized light was less than 45% between about 1.530 and 1.565
In another aspect of the present invention, the above-described grating membrane mirror provides an output-coupling mirror in a tunable OPS-laser. The OPS-laser includes a multilayer structure including a first mirror surmounted by a multilayer semiconductor gain-structure. The gain-structure has a gain-bandwidth including the lasing wavelength, and is energized by pump light having the pump-light wavelength. The inventive grating membrane mirror is peripherally supported by one surface of a semiconductor substrate over an aperture therein and is electrically isolated from the substrate. The mirror is spaced apart from the first mirror to form a laser resonator. The laser resonator has a longitudinal axis and the gain-structure is included in the laser resonator. Pump light is directed into the gain-structure through the grating membrane mirror. The optical spacing between the first mirror and the grating membrane mirror is selected such that the laser resonator supports only a single lasing mode, the wavelength of which is within the gain-bandwidth of the gain-structure.
At least one layer of the grating membrane mirror has an electrically conductive portion and means for making electrical contact with that electrically conductive portion. A electrical contact is provided a surface of the substrate opposite the surface supporting the grating membrane mirror. Applying an electrical potential between the electrically conductive portion of the grating membrane mirror layer and the electric contact on the substrate causes a central portion of the grating membrane mirror to move in a direction parallel to said longitudinal axis of said laser resonant cavity for selecting the wavelength of said lasing mode.
The first material, any second material, the grating width and height, the ratio between the grating width and period, and the thickness of the at least one and any other uniform-thickness layers are selected such that the second mirror has a sufficiently different reflectivity in the polarization planes oriented parallel and perpendicular to the grating strips, for light having the oscillating wave wavelength, that the lasing mode is plane polarized in the plane for which the reflectivity is highest, and such that the grating membrane mirror has a transmissivity greater than about 50% (0.50 in decimal terms) at said pump light wavelength in any polarization plane orientation.
As noted above the central optical telecommunications wavelength range extends from 1.535 xcexcm to 1.565 xcexcm. The gain-bandwidth of any particular semiconductor gain structure having a nominal emitting wavelength in this range is about 0.035 xcexcm. In a telecommunications system there can be a single laser in accordance with the present invention tunable over a gain bandwidth of about 0.035 xcexcm about the nominal wavelength. In all calculated examples of embodiments of the present invention discussed hereinbelow, a tuning wavelength range between 1.530 and 1.565 xcexcm is assumed. This is done simply to assist in comparing performance of the various examples and should not be construed as limiting the present invention.